Known capacitive sensors typically include a sensor electrode disposed on a dielectric substrate and a control circuit connected to the sensor electrode. A panel or substrate made of glass, plastic, or another suitable dielectric material may overlie the sensor electrode and define a touch surface overlying the sensor electrode.
The control circuit provides an excitation voltage to, and thereby generates an electric field about, the sensor electrode. This electric field establishes a capacitance, sometimes referred to as a parasitic capacitance, from the sensor electrode to ground or another reference potential (the terms “ground” and “other reference potential” may be used interchangeably herein). Introduction of a stimulus, for example, a user's finger, to or near the sensor electrode or corresponding touch surface establishes an additional capacitance, sometimes referred to as a finger capacitance, from the electrode, through the finger, to ground, thereby changing the overall sensor electrode-to-ground capacitance.
The control circuit also detects and monitors the sensor electrode-to-ground capacitance. The control circuit uses this data in conjunction with predetermined criteria to deem whether or not a touch event or proximity event has occurred. The control circuit could be configured to deem a touch event to occur when a stimulus touches the sensor electrode or corresponding touch surface. The control circuit could be configured to deem a proximity event to occur when the stimulus is near but not touching the sensor electrode or the touch surface. The control circuit could be configured to deem touch or proximity events to occur under other circumstances, as well, as would be understood by one skilled in the art.
The control circuit could be configured to deem whether or not a touch event or proximity has occurred by comparing the sensor electrode-to-ground raw capacitance at any given time, or for any given sample of raw capacitive data points, to a predetermined threshold and/or by comparing the raw capacitance to a baseline raw capacitance. The baseline raw capacitance typically would be a time-averaged measure of raw counts when no stimulus is near or proximate the sensor electrode.
For example, the control circuit could be configured to deem a touch event or proximity event to occur when the raw count exceeds or falls below a predetermined threshold. Alternatively, the control circuit could be configured to deem a touch event or proximity event to occur when the raw count deviates from the baseline raw count by at least a predetermined amount or difference.
In some embodiments, the control circuit could be configured to recognize and distinguish between both touch and proximity events. For example, the control circuit could be configured to deem a proximity event to occur when the raw count exceeds or falls below a first predetermined threshold or when the raw count deviates from the baseline raw count by at least a first predetermined difference. The control circuit also could be configured to deem a touch event to occur when the raw count exceeds or falls below a second predetermined threshold or when the raw count deviates from the baseline raw count by at least a second predetermined difference. Typically, the raw count would deviate from the baseline by a greater amount in response to touch of the stimulus to the sensor electrode or touch surface compared to mere proximity of the stimulus to the sensor electrode or touch surface. As such, the first predetermined threshold or difference typically would lie between the baseline and the second predetermined threshold or difference.
Conventional capacitive touch systems may have certain drawbacks. For example, the control circuits typically need to be tuned for a particular touch scenario. For example, they typically need to be tuned to detect touch by or proximity of a bare finger versus touch by or proximity of a gloved finger. Touch by or proximity of a bare finger typically would result in a greater finger capacitance and, therefore, a greater change in raw capacitance versus the baseline raw capacitance, than would touch by or proximity of a gloved finger. Indeed, the change in raw capacitance resulting from touch by a bare finger could be twice (or more or less) than the change in raw capacitance resulting from touch by a gloved finger.
As such, a system that is tuned to reliably detect touch by or proximity of a bare finger might not be sufficiently sensitive to detect touch by or proximity of a gloved finger. That is, touch by a gloved finger might not yield a change in raw counts exceeding the touch or proximity threshold. Although a system can be tuned to respond to touch by a gloved finger, a system so tuned might be overly sensitive to a bare hand such that a touch is deemed to have occurred when the bare hand is merely proximate but not touching the touch surface. A system so tuned also might be unacceptably susceptible to falsely “detect” touch when touch has not occurred because of noise or the presence of water, other contaminants, or spurious stimuli proximate or in contact with the sensing electrode or touch surface.